1. Field of the Invention
The present invention relates to a nitride semiconductor light-emitting element and a method for producing a nitride semiconductor light-emitting element.
2. Description of the Background Art
AlN, GaN, and InN have bandgaps of about 6.0 eV, about 3.4 eV, and about 0.6 eV, respectively. Therefore, a nitride semiconductor light-emitting element having these mixed crystals as an active layer can emit light having a wavelength region extending from the ultraviolet to the infrared, and it is being applied to various purposes such as a light source for lighting, or a backlight of a liquid crystal display.
A nitride semiconductor light-emitting element having an InGaN-based nitride semiconductor as an active layer can emit light having a wavelength region longer than a near-ultraviolet region. However, it is known that internal quantum efficiency of the nitride semiconductor light-emitting element is lowered as a wavelength of emitted light is increased, that is, an In composition is increased. This is caused by following reasons.
Compared with a nitrogen atom having extremely small ion radius, a lattice mismatch rate between InN and GaN composed of an In atom having a relatively large ion radius and a Ga atom having an intermediate ion radius, is about 10%, and a lattice mismatch rate between InGaN and GaN is 0% (GaN) to 10% (InN), depending on an In composition of InGaN.
Due to the lattice mismatch between InGaN and GaN, a crystal growth to uniformly distribute atoms in a crystal without disturbing an order of a crystal structure is more difficult to perform in a nitride semiconductor having a high In composition. Therefore, due to a lattice constant difference between InGaN and GaN, strong strain is applied to an InGaN/GaN quantum well active layer, so that a large internal electric field caused by piezoelectric polarization is generated in the active layer. It is considered that the internal quantum efficiency of the nitride semiconductor light-emitting element is reduced due to the internal electric field generated in the active layer.
For example, Patent Document 1 (Japanese Patent Laying-Open No. 2008-244360) discloses, in order to suppress phase separation in an InGaN active layer, a nitride semiconductor light-emitting element having an active layer between an n-type layer and a p-type layer, and having an optical confinement layer including an InGaN superlattice in which lattice relaxation is not provided at each of an interface between the n-type layer and the active layer, and an interface between the active layer and the p-type layer.
In addition, Patent Document 2 (Japanese Patent Laying-Open No. 1-222431) discloses an effect of restraining propagation of dislocation by a strained superlattice in a GaAs-based semiconductor element.
Furthermore, Patent Document 3 (Japanese Patent Laying-Open No. 6-188447) discloses a rapid lattice relaxation action by an InAsXP1-X/InAsYP1-Y superlattice in a GaInAs photodiode.